Switch to an AST based parser and builder

Starts using visionmedia/css-parse and visionmedia/css-stringify for
parsing the source and building the results. This avoids the messy and
hard to modify RegExes used to parse the source and makes it easier to
address issues like #19 or #18, adds the ability to add source maps and
fixes #8.
Ignore comments in property names and values
Ignore comments in declarations
Merge pull request #26 from ded/ast

Switch to an AST based parser and builder